Carlingford Castle (also known as King John's Castle) is a ruin that towers over the harbour of the town of Carlingford in County Louth. The castle was built around 1190 by the Norman baron Hugh de Lacy. The name "King John's Castle" derives from the visit of the English King John Lackland, who is said to have stayed there for three days in 1210. The castle played a role during the Battle of the Boyne (1690), where it was used as a hospital for the wounded soldiers of William of Orange.